This invention relates methods of making to thermistor chips with reduced fluctuations in the normal-temperature resistance values.
As shown in FIGS. 11 and 12, a conventional thermistor chip 1 of this kind usually has terminal electrodes 3 provided at both end parts of a thermistor block 2 having an oxide of a transition metal such as Mn, Co and Ni as its principal component. The terminal electrodes 3 each comprise an end electrode 3a formed by applying Ag/Pd or the like in a paste form and then firing and a plating layer 3b formed on its surface by using Ni or Sn. The normal-temperature resistance value (hereinafter simply referred to as "the resistance value") of such a thermistor chip is generally determined by the resistor value of the thermistor block 2 and the position of the terminal electrodes 3.
Fluctuations in the position of the end electrodes 3a, or more particularly in their width d and their separations a, are generally large because they are produced by applying a paste and firing. The so-called "3 cv" value (an index of fluctuations defined as 100.times.3.sigma./(average value) where .sigma. indicates the standard deviation of fluctuations in a lot) for the resistance values is conventionally as large as 5-20%. In order to reduce it to less than 1%, as currently becoming required of the resistant values, a sorting process is necessary, and this not only affects the production cost adversely but also makes it difficult to supply a large quantity of products.
Although it has been known to make use of a laser to remove a portion of the terminal electrode 3 so as to adjust the resistance value of a thermistor chip, the use of a laser involves several problems of its own such as damage to the thermistor block due to the laser heat. In the case of a thermistor of which the resistance changes nonlinearly with respect to temperature, the adjustment of the temperature value is difficult as the temperature of the thermistor block rises due to the laser heat.